Immunosuppression in Sydney rock oysters (Saccostrea glomerata) and QX disease in the Hawkesbury River, Sydney

This study assessed links between host fitness, environmental change and opportunistic parasite infections in a dynamic estuary system. The Hawkesbury River in New South Wales is the most recent Sydney rock oyster growing area to experience outbreaks of infectious QX disease. This area was used to examine a relationship between the intensity of QX disease and inhibition of the oyster immune system. Oysters were grown at various sites along the river and periodically monitored for general condition, total haemolymph protein content, antibacterial capacity and phenoloxidase activity. Phenoloxidase activity was significantly inhibited during a key period of Marteilia sydneyi infectivity in late summer 2005. The degree to which phenoloxidase was inhibited strongly correlated with the intensity of M. sydneyi infection. The data suggest that the presence of some transient environmental stressor may have affected phenoloxidase activity during a key period of infection and increased the susceptibility of oysters to disease. These results provide further evidence for a specific relationship between decreased phenoloxidase activity and susceptibility to QX infection.